1. Field of the Invention
The present invention relates to a spindle motor, of a disk apparatus, for rotating a disk and to a magnetic disk apparatus using this motor.
2. Description of the Related Art
In recent years, with the spread of the portable note-like personal computer, a magnetic disk apparatus is utilized as a file apparatus mounted in this machine. Down-sizing, a reduction in thickness and low dissipation of power the magnetic disk apparatus mounted in this type of machine are strongly demanded. For thinning the magnetic disk apparatus, it is demanded that a spindle motor for rotating the magnetic disk be reduced in height.
FIG. 7 is a view of the prior art to aid in the following explanation.
The present applicant made an application for a technology of thinning the spindle motor (Specification of Japanese Patent Application No. 4-220379, filed on Aug. 19, 1992). This spindle motor will be described with reference to FIG. 7.
The numeral 1 designates a housing for a magnetic disk apparatus. A spindle shaft 2 is secured to upper and lower surfaces of this housing 1. A spindle hub 5 is rotatably attached to this spindle shaft 2 via an upper bearing 3 and a lower bearing 4. A spacer 6 holds a spacing between the upper bearing 3 and the lower bearing 4.
A diameter of a flange 5-1 of the spindle hub 5 is slightly smaller than a minor diameter of a magnetic disk 7. An innermost peripheral portion of the magnetic disk 7 is secured by bonding onto an outer cylindrical member 5-2 of the spindle hub 5 with being guided by the flange 5-1.
A ring-like permanent magnet 8 is mounted the interior of the spindle hub 5. A plurality of stators 9 are bonded to the housing 1. An armature coil 10 is wound on a proximal portion of each of these stators 9. This armature coil 10 is connected to a printed circuit board 11 and supported thereon.
The magnetic field system of this spindle motor will be explained next. The spindle hub 5 is composed of a non-magnetic material such as aluminum, etc. The permanent magnet 8 is composed of a ring-like polar anisotropic magnet. That is, the permanent magnet 8 is, e.g., 8-pole-magnetized outward in the radial direction of the magnet. The housing 1 and the stators 9 are constructed of soft steels and form a magnetic path (yoke). Accordingly, the magnetic flux of the N-pole of the permanent magnet 8 and the S-pole adjacent thereto passes through the interior of the magnet, the stator 9 and the housing 1.
Next, the operation of the construction discussed above will be described. An unillustrated control circuit sequentially flows the electric current across the armature coils 10, and the stators 9 are thereby turned out electromagnets. Accordingly, driving forces are produced in the magnetic gaps between the permanent magnet 8 and the stators 9, thereby rotating the permanent magnet 8. That is, a motor of the brushless DC motor type is formed. With this operation, the spindle hub 5 is rotated about the spindle shaft 2, and the disk 7 is rotated at the desired number of revolutions.
There arise, however, the following problems inherent in the prior art.
Generally, the dissipation power at the predetermined number of revolutions is proportional to the ratio of the load torque loss to the torque constant. Namely, the dissipation power is expressed by the following formula:
Dissipation Power=Power Supply Voltage.times.Dissipation Current=Power Supply Voltage.times.(Load Torque Loss/Torque Constant) PA1 S=.pi..times.Magnet Average Radius.times.Magnet Height/Number of Poles
Accordingly, it is required for reducing the dissipation power of the motor that the motor be constructed to increase the torque constant. Also, the load torque loss is the total sum of the windage loss (loss caused when the rotor rotates through the ambient air), a mechanical loss (frictional loss in the bearing) and the iron loss (eddy-current loss and hysteresis loss).
This torque constant is proportional to the number of conductors per armature coil and the quantity of the magnetic flux produced by the magnetic field system. That is: Torque Constant.varies.Number of Conductors.times.Magnetic Flux Quantity. The quantity of the magnetic flux produced by this magnetic field system, simply, increases in proportion to the equivalent sectional area S of the permanent magnet 8. The equivalent sectional area S of the permanent magnet 8 is given by the following formula:
The above-mentioned motor based on the conventional construction, however, presents the following problems. For attaining the down-sizing and the reduction in thickness, the permanent magnet 8, the stators 9 and the armature coils 10 are arranged inwardly of the flange 5-1, on the spindle hub 5, which guides the disk 7. With this arrangement, it is difficult to increase an average diameter of the permanent magnet 8. Hence, the quantity of the magnetic flux produced by the magnetic field system is also small. This makes it difficult to produce a motor exhibiting a large torque constant while requiring a lower quantity of dissipation power.